Origin of low δ26Mg Cenozoic basalts from South China Block and their geodynamic implications

Iron and magnesium isotopic compositions of subduction-zone fluids and implications for arc volcanism

Extreme Nb/Ta fractionation in metamorphic titanite from ultrahigh-pressure metagranite

Mobilities and fractionations of high-field-strength elements, especially Nb and Ta within a subducting slab, are important for deciphering the formation of the continental crust (CC). Here we report geochemical results on an epidote garnet amphibolite facies metagabbro body in the Tongbai-Dabie orogenic belt, central China. Our samples were hydrated during prograde metamorphism of the Triassic plate subduction. Major minerals such as amphibole, garnet, rutile, and ilmenite and garnet amphibolite bulk rocks show varied and overall lower Nb/Ta and/or Zr/Hf ratios than the continental crust. Magma differentiation might have contributed to variations of Zr/Hf but not those of Nb/Ta, suggesting major Nb/Ta fractionations during plate subduction. LA-ICPMS in situ trace element analyses of amphibole and especially rutile grains exhibit obvious chemical zonations. Typically, the rutile cores are usually small with higher Nb and Ta concentrations and lower Nb/Ta ratios compared to the thick rims. Chemical...

https://www.jstor.org/stable/10.1086/669978

Major Nb/Ta Fractionation Recorded in Garnet Amphibolite Facies Metagabbro

U–Pb ages and trace elements of metamorphic rutile from ultrahigh-pressure quartzite in the Sulu orogen

Niobium and Ta concentrations in ultrahigh-pressure (UHP) eclogites and rutile from these eclogites and associated high pressure (HP) veins were used to study the behaviour of Nb-Ta during dehydration and fluid-rock interaction. Samples were collected through a � 2 km profile at the Bixiling complex in the Dabie orogenic belt, Central-Eastern China. All but one eclogite away from veins (EAVs) display nearly constant Nb ⁄ Ta ratios ranging from 16.1 to 19.2, with an average of 16.9 ± 0.8 (2 SE), similar to that of their gabbroic protolith from the Yangtze Block. Nb ⁄ Ta ratios of rutile from the EAVs range from 12.7 to 25.3 among different individual grains, with the average values close to those of the corresponding bulk rocks. These observations show that Nb and Ta were not significantly fractionated by prograde metamorphism up to eclogite facies when no significant fluid-rock interaction occurs. In contrast, Nb ⁄ Ta ratios of rutile from eclogites close to veins (ECVs) are highly variable from 17.8 to 49.8, which are systematically higher (by up to 17) than those of rutile from the veins. These observations demonstrate that Nb and Ta were mobilized and fractionated during localized fluid flow and intensive fluid-rock interaction. This is strongly supported by Nb ⁄ Ta zoning patterns in single rutile grains revealed by in situ LA-ICP-MS analysis. Ratios of Nb ⁄ Ta in the ECV-hosted rutile decrease gradually from cores towards rims, whereas those in the EAV-hosted rutile are nearly invariable. Furthermore, the vein rutile shows Nb ⁄ Ta zoning patterns that are complementary to those in rutile from their immediate hosts (ECVs), suggesting an internal origin for the vein-forming fluids. The Nb ⁄ Ta ratios of such fluids evolved from low values at the early stage of subduction to higher values at later supercritical conditions with increased temperature and pressure. Quantitative modelling was conducted to constrain the compositional evolution of metamorphic fluids during dehydration and fluid-rock interaction focusing on Nb-Ta distribution. The modelling results based on our proposed multistage fluid phase evolution path can essentially reproduce the natural observations reported in the present study.

Nb–Ta fractionation induced by fluid‐rock interaction in subduction‐zones: constraints from UHP eclogite‐ and vein‐hosted rutile from the Dabie orogen, Central‐Eastern China

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Nb–Ta fractionation induced by fluid‐rock interaction in subduction‐zones: constraints from UHP eclogite‐ and vein‐hosted rutile from the Dabie orogen, Central‐Eastern China